The invention is in general directed toward a gas sensor and in particular to a miniaturized gas sensor with film type electrodes and a solid ionomer electrolyte.
Film based techniques have been investigated for a wide variety of sensors, as reported by Wenyi et al., 1997; Hughes et al., 1997; Staley, 1996; Agbor et al., 1995; Tan and Tan, 1995; Menil et al., 1994; Kunnecke et al., 1994; Creasey and Varey, 1994; Geistlinger, 1993; Ishiji et al., 1993; Najafi et al., 1992; Hampp et al., 1992; Nakano and Ogawa, 1994; and Yamazoe and Miura, 1994. While solid-state gas sensors have the advantage of being able to operate at elevated temperatures, they also have the disadvantages of slow response and recovery time and a high internal operating temperature as reported by Liu et al., 1993; Narducci et al., 1993 and more recently by Schwebel et al., 1997; Sheng et al., 1997; and Micocci et al., 1997. Substantial development work needs to be done with this type of sensors before they can be utilized in battery-powered sensor instruments.
A Nafion(copyright)-coated metal oxide pH sensor was reported (Kinlen et al., 1994) with sputtered iridium oxide sensing and silver/silver chloride reference electrodes on alumina ceramic substrates. Nafion was used as a cation-selective ionomer coating in order to decrease the oxidation-reduction error generally affecting the performance of metal oxide pH electrodes. The use of Nafion as a polymer-electrolyte for a thin-film CO sensor was described (Yasuda et al., 1994) with macro-sized, sputtered Pt sensing electrodes and counter electrodes and a smaller, sputtered Au electrode as a reference electrode. A 5 wt % n-propyl alcohol solution of Nafion (DuPont, 1100 EW) was used to form the polymer electrolyte film over the electrodes by casting. The polymer was washed and protonated in aqueous sulfuric acid prior to casting. The lifetime of this sensor was reported to be less than one month. During this one month lifetime the CO oxidation current decreased steadily down to a few percent of its original value without any period of stable measurement signal. The lifetime of the device may be extended by up to three years by lamination of the polymer electrolyte layer with a cast perfluorocycloether-polymer film in order to keep the CO permeability coefficient through Naflon constant. Theoretical calculations showed that the drift rate of the signal could be significantly reduced under these conditions.
Descriptions of typical state-of-the-art hydrated solid polymer electrolyte or ionomer sensors and sensor cells are provided by Kosek et al. U.S. Pat. No. 5,527,446; LaConti and Griffith, U.S. Pat. No. 4,820,386; Shen et al., U.S. Pat. No. 5,573,648; and, Stetter and Pan, U.S. Pat. No. 5,331,310. These sensor cells, based on hydrated solid polymer electrolyte or ionomer technology, have several advantages over conventional electrochemical sensor cells. The catalytic electrodes are bonded directly to both sides of a proton conducting solid polymer ionomer membrane providing a stable electrode to electrolyte interface. One side of the electrolyte membrane is flooded with distilled water, making the sensor cell self-humidifying and independent of external humidity. Since no corrosive acids or bases are used in the sensor cell, over 10 years lifetime has been demonstrated for solid polymer ionomer sensor cells. Finally, the sensor cells are easy to maintain, thus ideal for use in remote, unattended environments. Regular addition of water to the reservoir in the sensor housing every several months, and monthly calibration checks are the only requirements.
A disadvantage of the state-of-the-art sensors described above is that the signal-to-noise ratio may not be conducive to the detection of very low concentrations (parts per billion, ppb) of important environmental and biomedical gases and vapors. Also, response time may be relatively slow, and reproducibility between sensors and sensor cells may be difficult to achieve. The sensors are also relatively costly.
The objective of this invention is to overcome the present limitations of miniaturized electrochemical sensors by uniquely interfacing advanced solid polymer ionomer membrane configurations with film type electrode structures to obtain low maintenance, highly sensitive, rapidly responsive, reproducible, sensor devices for environmental, industrial, and biomedical monitoring. By using a uniquely designed film type electrode array in intimate contact with an advanced solid polymer ionomer membrane film configuration, to form a three-phase contact area for the sensing electrode, where the gas sample, the electrode, and the solid ionomer can interface, a superior signal-to-noise ratio, rapid response time, and reproducibly of at least 1 to 10 ppb for a selected gas in an ambient environment can be achieved. Also, the projected cost of these sensors and sensor cells is very low since established film type solid-state manufacturing processes can be utilized.
The invention is also directed toward a treatment process for catalytically activating an ionomer membrane.
The invention is still Further directed toward a gas sensor utilized in conjunction with a gas sensor control circuit.
The invention is also directed toward a gas sensor utilized in a gas sensing instrument.